Synchronous linear motors generally comprise a carriage having permanent magnets, with the carriage being mounted for movement adjacent to a primary made of ferromagnetic material and which may be electrically energized to also interact with the synchronous secondary, or other propulsion means not associated with the synchronous secondary may be employed.
A description of such a synchronous linear motor is illustrated in commonly owned U.S. patent application Ser. No. 115,791 filed Nov. 2, 1987, now U.S. Pat. No. 4,825,111, and the disclosure therein is incorporated herein by reference. The linear motor carriage will usually carry a tool to perform some type of work while passing through a work station.
In certain applications of synchronous linear motors, it is necessary for the primary to be discontinuous and have discrete ends. When this is the case, the linear motor carriage with attached synchronous secondary must approach the leading end and leave the trailing end of the primary. The carriage may be separately propelled by contact with an adjacent carriage or propulsion by a second attached synchronous secondary. Approaching and leaving the ends of the primary creates a problem because of the change in force acting on the secondary at the ends of the primary. The primary consists of ferromagnetic material, usually steel laminations, that interact magnetically with the magnetic poles of the synchronous secondary. The secondary poles are often provided by permanent magnets. As the magnets of the secondary are moving through the space preceding the primary and then approach the leading end of the primary, they are attracted strongly to the ferromagnetic end of the primary, so a large force unbalance exists on the secondary. This strong attraction may make the secondary motion speed up if there is insufficient restraint on the secondary. When the secondary reaches the leading end of the primary it comes under the influence of any moving electromagnetic wave in the primary. If the speed of the EM wave is different from the speeded up motion of the secondary, a disturbance in the motion of the secondary results. This is often the case and is frequently undesirable. The secondary magnet is also attracted to the rectangular horizontal surface of the primary while moving between its ends, but this results in a balanced force on the secondary which has no effect on the speed of the secondary.
The opposite effect occurs as a synchronous secondary leaves the trailing end of the primary. As the magnet tries to move away from the trailing ferromagnetic end, an unbalanced force on the magnet exists and the secondary motion is slowed down excessively if there is no driving force on the secondary. Once again, a disturbance in the motion of the secondary often results and is frequently undesirable.
In some cases, a moveable structure must be provided in a linear motor track to allow the carriage to change direction. At the points of movement, it is not practical to provide a continuous linear motor primary structure or other controllable propulsion means for the linear motor carriage. The carriage with synchronous secondary attached must be moved between portions of discontinuous propulsion means which do not interact with the synchronous secondary. Some simple economical means of alternate propulsion for the carriage must be provided to move the carriage from one controllable propulsion means to another.